The use of heliostats in the fields of concentrating solar power and concentrating photovoltaics is well established in the prior art. A typical CSP or central tower CPV system includes a centralized tower and a plurality of suitably mounted heliostats that are pointed so that the reflected sunlight impinges on the central tower focal point which often is fixed in space relative to the heliostat. For such conditions to be realized, the laws of reflection require that the angle formed between the sunlight vector and the mirror normal must be equal to the angle formed between the properly aimed redirected ray of light and the mirror normal. All three vectors also must lie in the same plane. It can be shown using vector algebra that for a given a sunlight vector and a properly aimed reflected light ray, there is a unique solution for the mirror normal that is simply the normalized average of the incident and reflected light vectors.
The centralized tower serves as the focal point onto which individual heliostats redirect sunlight. The aim of the heliostats can be controllably adjusted as the sun moves in the sky to redirect incident sunlight onto the centralized tower. The concentration of sunlight at the tower focus is therefore correlated to the number of heliostats up to certain fundamental limits. The high concentration of solar energy is converted by the tower into other useful forms, such as heat which can then be used either directly or be used to generate steam to power electrical generators, or directly to electricity through the use of any number of photovoltaic devices generally referred to as solar cells.
Heliostats generally include a mirror to redirect sunlight, support structure to hold the mirror and to allow the mirror to be articulated, and actuators such as motors to effect the articulation. At a minimum, heliostats usually provide at least two degrees of rotational freedom in order to redirect sunlight onto a fixed tower focus area. Heliostat mirrors are often planar, but may also have more complex shapes.
Heliostat articulation can follow an azimuth/elevation scheme by which the mirror rotates about an axis perpendicular to the earth surface for the azimuth and then rotates about an elevation axis that is parallel to the earth surface. The elevation axis often is coupled to the azimuth rotation such that the direction of the elevation is a function of the azimuth angle. Alternatively, heliostats can articulate using a tip/tilt scheme in which the mirror rotates about a fixed tilt axis that is parallel to the earth surface. The tip axis is orthogonal to the tilt axis but its direction rotates as a function of the tilt axis. The tip axis is parallel to the earth surface when the heliostat mirror normal vector is parallel to the normal of the earth surface. Other schemes, such as polar tracking and many others, are also possible; the present invention is applicable to any of these schemes.
Many heliostat control systems employ open loop algorithms based on system geometry and sun position calculators in order to determine the sun and heliostat-to-focus vectors as a function of time. These calculations result in azimuth/elevation or tip/tilt commands to each heliostat device. Such control systems generally assume that the location of the heliostats are static and well defined, or otherwise rely on periodic calibration maintenance to correct for settling and other lifetime induced drifts and offsets. Open loop solutions are, advantageous in that they don't require any specialized feedback sensors to detect how well each heliostat is pointed. These systems simply tell every heliostat how to point and assume that the heliostats point correctly.
Closed loop heliostat pointing, on the other hand, relies primarily on feedback from one or more sensors capable of measuring differences between the desired heliostat-to-focus vector and the actual reflected sunlight vector. These errors are then processed into compensation signals that are provided to the heliostat actuators to articulate the mirror so that reflected sunlight impinges on the tower focus. Closed loop pointing has an advantage that it does not require precise installation or knowledge of the system geometry and can be made less sensitive to lifetime drifts. The elimination of the precision installation normally associated with open loop systems is a noteworthy advantage of closed loop pointing.
A difficulty in applying closed loop pointing methods on CSP systems results from the pointing condition requiring the bisection of two vectors rather than alignment to a single vector. That is, during normal operation, a heliostat mirror itself doesn't point at anything in particular. Rather, it must point in a direction between the sun and the target, and the point moves with time as the sun moves. Nominally, there is nothing in that direction but empty sky, so there is nothing for a traditional closed loop tracking system to point the mirror at.
The ideal closed loop heliostat tracking system could directly sense the difference between the actual reflected sunlight vector and the vector corresponding to a properly aimed reflected light vector, and then endeavor to control that difference to zero. Other schemes are possible, albeit less desirable.
For example, one prior art system (http://www.heliostat.us/howitworks.htm) discloses a sensor that controls the sunlight vector to be aligned with a third vector, which is the axis of a sensor near the heliostat. During installation of the system, the sensor is aligned with the line of sight vector to the target. The, accuracy of the system is thus dependent on the accuracy of this alignment and on the alignment remaining unchanged. In large CSP systems, however, this may be insufficient for several reasons. For example, the tower may sway in the wind or experience thermal expansion or contraction. Components can wear or shift. Cost may also be an issue, since each heliostat requires a separate sensor.
A second type of “closed loop” heliostat system that is common in the prior art is a system that senses the orientation of the heliostat axes with respect to the heliostat base. The control system then provides corrections to any detected errors in the orientation of these axes. This type of system mitigates errors in the gear train of the heliostat or errors, but it does not sense the sunlight vector at all. Consequently, this approach is susceptible to unseen errors in this vector, and it is blind to any errors in the alignment of the sunlight vector to the line of sight to the target. This system thus likewise may be sensitive to motions of the tower and long-term drifts. Practical systems tend to include elaborate calibration schemes to deal with these issues. Cost also is impacted, since encoders are needed for each axis of each heliostat.
As an alternative, a closed loop system more desirably would observe and track the reflected sunlight beam directly. An obvious location for a feedback sensor would be at the tower focus, since this is where the reflected beam is targeted. Generally, this is not practical for systems that concentrate substantial sunlight, because no practical sensor could withstand the extreme temperatures or the ultraviolet radiation that is present at the focus. Nonetheless, it is desirable to control the difference between the reflected sunlight beam and line of sight corresponding to a reflected beam that is properly aimed. Consequently less direct methods of providing feedback would be desired to make closed loop pointing feasible.
The use of optical proxies is an innovative approach that makes it feasible to use closed loop pointing. Using optical proxies to help control the aim of heliostats is described in U.S. provisional applications No. 61/562,962, entitled “Optical Proxy for Sensing and Pointing of Light Sources”, filed Nov. 22, 2011 and No. 61/465,165, entitled “Apparatus and Method for Pointing Light Sources,” filed Mar. 14, 2011, WO 2012/125748A2, WO 2012/125751A2, and these applications and publications are incorporated here in their respective entireties by reference for all purposes.
The aforementioned applications introduced the use of one or more optical proxies, such as diffractive or diffusive elements, having optical characteristics that correlate to the aim of the heliostat and therefore also correlate to the actual vector of the primary light beam reflected from a heliostat. Optical characteristics of the proxies, not the reflected light beam, are sensed using a plurality of imaging sensors to allow indirect sensing of the actual aim of the reflected light beam. If the actual aim deviates from the desired aim, the aim is corrected. Multiple imaging sensors viewed one or more of the optical proxies from different perspectives in a manner such that the optical characteristics of the one or more proxies varied from perspective to perspective. In a manner analogous to the way multiple variables can be solved using multiple equations, the plurality of unique perspectives of sensed information allowed the vector of the reflected light to be accurately determined even though the at least one proxy, not the reflected light, was being sensed. This advantageously permitted closed-loop control of the aim of the reflected light beam. This technique is especially useful in the fields of concentrating solar power (CSP) or central tower concentrating photovoltaics (CPV) for controlling heliostats.
Numerous techniques are possible for using information from optical proxies to determine the aim of the redirected light beam. The aforementioned applications detailed one of these techniques that involved using multiple imaging sensors to observe the one or more proxies from multiple perspectives so that the vector of the reflected light could be uniquely determined. An approach that permits the use of a smaller number of perspectives, including as few as one perspective, would also be desired.